It has been estimated that 15-20 percent of all human conceptions are chromosomally abnormal (aneuploid), and that many of these abnormalities occur because of errors in chromosome segregation during female meiosis. One of the strongest predictors of aneuploidy is maternal age with nearly half of all oocytes ovulated in women over age 40 containing errors in chromosome number. Most aneuploid conceptions die before birth, but several specific aneuploid states (e.g. trisomy 21) are viable and, as a class, represent the most common cause of mental retardation in man. Despite this clinical importance, very little is known about the cause of chromosome nondisjunction during female meiosis or why the rate of non-disjunction increases with maternal age. One model proposed to explain the correlation between maternal age and frequency of chromosome nondisjunction suggests that the efficiency of folliculogenesis (the development of the oocyte within the follicle in the ovary) is reduced in older women. We hypothesize that reduced or impaired folliculogenesis could contribute to chromosome non-disjunction by reducing the quantities of specific microtubule motor proteins necessary for alignment and/or segregation of chromosomes on the meiotic spindle. To test this hypothesis, we propose to use transgenic mouse technology to specifically perturb the function of the kinesin-related proteins Kid and MCAK during female meiosis. We are targeting Kid and MCAK because they are not necessary for overall spindle assembly, but are required for efficient chromosome alignment and/or segregation during mitosis and, in some non-mammalian systems, meiosis. We will specifically perturb the function of Kid or MCAK during female meiosis by transgenic expression of anti-sense or dominant negative constructs using a mouse oocyte-speciflc promoter. We will evaluate the consequences of disrupted motor function during female meiosis by examining the fertility of transgenic females, the frequency and type of birth defects in pups born from transgenic females, and the alignment of chromosomes on meiotic spindles in oocytes from transgenic females. Successful completion of these experiments will validate the application of transgenic mouse technology to study the mechanisms of spindle assembly during female meiosis in mammals and could increase our understanding of the etiology of disorders such as Down?s Syndrome.